1. Field of the Invention
The present invention relates to an image processing system employed in, for example, a color printer, a color copy machine, a color display system or the like, for which a high color reproducibility is required, and a method thereof. More particularly, it relates to a color correction system with a relatively small capacity memory for storing color gradation corrected data and carrying out a mix of a looking-up from a look-up table and an interpolation, and a method thereof.
2. Description of the Related Art
In a color correcting method for a color printer or the like, a color masking processing is used. This color masking method corrects three kinds of original color gradation data, i.e., cyan data having a gradation C, magenta data having a gradation M, and yellow data having a gradation Y, supplied the color printer, to three kinds of color gradation corrected data, i.e., corrected cyan data having a gradation C', corrected magenta data having a gradation M' and corrected yellow data having a gradation Y', by using correcting parameters. For example, a first order masking algorithm is expressed by the following formula; ##EQU1## where, a.sub.11 to a.sub.33 denote correcting parameters.
A second order masking algorithm is expressed by the following formula. ##EQU2## where, C.sup.2, M.sup.2 and Y.sup.2 denote self products of the gradation of each three color, cyan, magenta and yellow input data,
CM, MY and CY denote mutual products among three color cyan, magenta and yellow input data, and PA1 a.sub.11 to a.sub.39 denote correction parameters.
A set of the three kinds of color gradation input data (C,M,Y) is previously calculated by the above masking formula (1) or (2) as a set of three kinds of color gradation corrected output data (C',M',Y').
As a prior method of color correction, a method is known wherein the color gradation corrected output data (C',M',Y') for all combinations of all color gradation data (C,M,Y) are previously calculated in accordance with the formula (1) or (2) and stored in a table memory, and a output of the color gradation corrected output data corresponding to the color gradation input data (C,M,Y) is carried out by a look-up in a look-up table. This method, however, requires a memory having a large capacity, for storing all of the previously calculated color corrected gradation data (C',M',Y') and thus suffers from a disadvantage of a bulky system and high cost. For example, when each color gradation is expressed by eight bits (one byte), to represent 0 to 255 gradations, the capacity of the table memory is 2.sup.24 .times.3 bytes (48 M bytes or 384 M bits).
To overcome the disadvantage set forth above, Japanese Examined Patent Publication (kokoku) No. 52-16493 discloses an apparatus in which, to reduce the size of the table memory, only a part of three kinds of color gradation corrected data previously obtained by the above-mentioned formula and used for looking-up in a look-up table is stored in the table memory, the remaining color correcting gradation data are not stored in the table memory, and the interpolation is carried out.
In the above-mentioned combination of looking-up in a look-up table and carrying out an interpolation method, the interpolation is carried out independently for each kind of color gradation. For example, when a cyan gradation corrected output data C' is to be obtained, the output data C' is interpolated between adjoining previously calculated cyan gradation corrected output data, as an intermediate data, i.e., an average of the adjoining cyan gradation correction output data. Specifically, first, the looking-up in the look-up table is carried out to find a set of previously calculated color gradation corrected data (C.sub.r,M.sub.r,Y.sub.r) positioned at the nearest coordinate position to the input gradation coordinate (C,M,Y), and another adjacent set of color gradation corrected data (C.sub.r+1, M.sub.r+1,Y.sub.r+1), and second, a one-dimensional interpolation of the cyan gradation data C is carried out for only the cyan, on the basis of the relationship among C, C.sub.r, C.sub.r+1. Namely, to interpolate the cyan element, the relationship of the magenta and yellow gradation data is not considered. Accordingly, when the input magenta gradation data M satisfies M.sub.r &lt;M&lt;M.sub.r+1 and the input yellow gradation data Y satisfies Y.sub.r &lt;Y&lt;Y.sub.r+1, the color gradation corrected data are equal. This also applies when obtaining other color gradation corrected data, i.e., magenta and yellow gradation corrected data M' and Y'. Namely, the method discussed above suffers from a disadvantage of a low accuracy color reproducibility. In addition, multiplying circuits are required for calculating, for example, where C is located at a position between C.sub.r and C.sub.r+1, and thus the cost thereof becomes high.
The above problems can occur in other systems, for example, a color display system in which a correction for gradations of three kinds of fundamental light elements, i.e., red, green and blue, is required.